In selecting tense, your
first concern is to avoid confusing the reader or whip-lashing the reader
with frequent or unclear changes in tense. In any section, try to establish
one tense as the main tense and then depart from that tense as logic requires.

In scientific writing,
you will usually use past tense when describing what others found and what
you found. For example, in an Introduction or a Discussion you might write:

But
use present tense when the finding is accepted as a general fact. You would
say:

'Plant
growth often depends on nitrogen availability', not

'Plant
growth often depended on nitrogen availability.'

Similarly,
you would say:

'Photosynthesis
requires water and carbon dioxide' , not

'Photosynthesis
required water and carbon dioxide.'

The
decision to use past or present tense requires judgement about the condition
of the statement - if the statement is widely accepted, you will probably
use present tense. If the statement reflects a particular investigator or
your own findings, use past tense to show that the finding may not always
apply.